In these industry branches, the need for robust, durable lightweight construction has increasingly promoted the use of fiber-reinforced composite (FRC) materials, especially carbon fiber composites (CFC), during recent years. Compared to metals, they usually feature a complex, layered structure and therefore anisotropic material properties leading to a variety of different possible defect types that need to be reliably identified. Consequently, the development of NDT techniques suitable for these materials, preferably allowing a high degree of automation to save costs and increase reliability as well as testing speed, is of great importance.
A number of NDT methods have been developed so far. Based on the underlying physical detection mechanism, they mostly fall into a few broad categories: Magnetic Particle Inspection, Inductive or Eddy Current Inspection, Visual and Optical Inspection, Radiography and Thermography, Ultrasonic, as well as Sonic Inspection and Shearography. The first category is limited to ferromagnetic materials and therefore unsuitable for the inspection of FRC, while Eddy Current Inspection is applicable specifically to CFC since carbon fibers are conductive even though the surrounding matrix is usually not. Testing procedures belonging to all other categories have been assessed and implemented for FRC. Comparative reviews of NDT techniques for composite materials show that none of these technologies are equally applicable to the detection of each of the different defect types common in FRC at this point. While all methods have their advantages and drawbacks, some methods are inherently limited to certain test object geometries. Visual and Optical Inspection methods, for instance (with or without colored or fluorescent liquid penetrant) are limited to the detection of surface defects, with the exception of visible defects in transparent materials. Sonic Inspection methods typically suffer from comparably low sensitivity and spatial resolution. Shearography and Thermography are limited to the detection of near-surface defects.
Ultrasound methods (ultrasonic methods) and Radiography, on the other hand, are in principle capable of detecting and localizing both surface and bulk defects with high spatial resolution and sensitivity. However, for FRC, Radiography based methods, including high-resolution CT imaging, are not well suited to the detection of delamination defects, which constitute one of the most common defect classes in FRC, without use of an X-Beam (or Gamma) absorbing liquid penetrant. This, however, leads to significant complication of the procedures and limits the method to the inspection of surfaces or surface-near layers penetrable for the contrast liquid. Further in many test environments liquids are not wanted or not accepted.
Due to their versatility and good sensitivity, various ultrasound techniques are routinely applied for the inspection of FRC. Most of them employ piezoelectric transducers for generation of ultrasound pulses, their detection or both. While many specific methods to provide detection, imaging or volumetric localization of defects have been developed, distinguished by the exact configuration and number of transducers, they usually fall into two different operation modes. One approach is through-transmission mode, where the test specimen is placed between two piezoelectric transducers, acting as transmitter and receiver respectively. In this configuration, the receiver detects the attenuation of the primary ultrasound pulse due to defects. This configuration poses constraints on the specimen shape and also thickness due to the pronounced attenuation of ultrasound in FRC compared to metals. Alternatively, the pulse-echo method can be employed, where reflection or backscattering of the primary pulse due to defects is detected from one side of the sample. This method significantly facilitates applicability to complex-shaped test objects of varying thickness—as access to the test object is sufficient from one side only.
Conventional ultrasound testing systems employ a medium such as water or an emulsion or gel respectively to guarantee good coupling of ultrasound pulses to the test object. Allowing the use of ultrasound frequencies up to ˜20 MHz, however, requiring immersion into a water basin or water jets between transducers and the specimen. Coupling fluids cannot be used for certain FRC structures or certain test environments, which creates need for non-contact test methods.
One actively developed option for contactless testing is the all-optical laser ultrasound method where an ultrasound pulse is created by absorption of a sufficiently strong laser pulse within the test specimen, and detection is performed interferometrically. A different approach is air-coupled ultrasound, similar to the conventional transducer-based method, but foregoing the coupling medium. This approach is enabled by increasingly sensitive highly resonant focused ultrasound transducers, allowing defect detection in spite of the decreased coupling caused by an air gap. While air-coupled systems using through-transmission mode are currently available, the implementation of an air-coupled pulse-echo configuration allowing one-sided tests of specimens poses significant problems. Highly resonant transducers oscillate for many periods both during pulse generation and detection, leading to a significantly increased “dead zone”. This term denotes the surface-near region of the test object where defect detection is rendered impossible due to overlap between primary pulse, reflections from the sample surface and the actual signal contributed by backscattering from defects. For that reason, the skilled persons refrain from using this test method for precise material testing.
A contactless weld mechanical joint sample quality test procedure uses a square wave modulated laser beam to excite an ultrasonic impulse wave in the sample and measures its pulse amplitude interferometrically after propagation through the joint. The laser is moved towards and away from the test object. For the detection interferometrical methods which are known per se are used. The mechanically moving surface of the test object is scanned optically or the detection is done by means of a prior art microphone, without the use of any thermo-acoustic effect.
A measuring device for non-mechanical-contact measurement of a layer is known in the art, the measuring device including a light source operative to generate a pulse adapted to interact with the layer so as to generate a thermal wave in a gas medium present adjacent the layer, what makes use of the photo-acoustic effect. The thermal wave causes an acoustic signal to be generated. The measuring device further includes a detector adapted to detect a first signal responsive to the acoustic signal, the detector not being in mechanical contact with the layer. The first signal is representative of the measured layer.
A further method and arrangement for non-destructive evaluation of materials allows for the inspection of products without damaging the material. A continuous wave high-power laser sweeps across the material, using thermo-elastic expansion to create an ultrasound wave front on the surface of and in the material. Detection of the ultrasound from the test piece can be achieved by different methods, providing area, line or point detection, respectively. Point detection, where a single data point is capture at a time, is the typical method used for laser ultrasonics. Contact transducers can be used, but generally an optical detection method is used. Different interferometers have been used, including heterodyne (two beam), confocal Fabry-Perot, and photo-refractive quantum wells. A probe laser beam is directed to the detection point on the sample. The reflected light is gathered in an interferometer and sensed by a photodetector. Surface displacement caused by the ultrasound changes the interference of the light, which creates the signal. The detection point can be on the same side as the generation laser (pulse-echo) or the opposite side (through transmission). All these methods rely on deformations or spatial movements of elements in the sound detectors or microphones. Even with the favored line detection, in particular by Gas-coupled Laser Acoustic Detection, the ultrasound is sensed by directing a laser beam through the acoustic disturbance.
The disturbance by physical displacement causes a change in the optical path of the beam that can be detected with a position-sensitive photodetector.